Channel estimation in OFDM transmission system and method

ABSTRACT

A system and method is provided for estimating the channel in OFDM transmission with inter-carrier interference (ICI). A channel in a data subcarrier in a subchannel shared between pilot subcarriers and data subcarriers can be estimated by performing interpolation based on estimated channels in pilot subcarriers in the same OFDM symbol as the subcarrier, such as through spline interpolation. A second estimate of the channel in the subcarrier can be produced by averaging an estimate of the channel in a subcarrier in the subchannel in a previous OFDM symbol and an estimate of the channel in a subcarrier in the subchannel in a succeeding OFDM symbol. A third estimate of the channel in the subcarrier can be produced through a linear combination of the first estimate and the second estimate. The channel in data subcarriers can be estimated through a weighted sum of the channel in nearest subcarriers.

COPYRIGHT NOTICE

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FIELD OF THE INVENTION

This invention relates generally to the field of wireless communication,and more specifically to channel estimation in OrthogonalFrequency-Division Multiplexing (OFDM) transmission.

BACKGROUND

In the age of information, the field of communications devices hasexperienced among the most rapid rates of innovation of any area oftechnology. As the requirements of communication devices continue toincrease, methods for faster, cheaper, and more reliable data transferare continuously developed. Orthogonal frequency-division multiplexing(OFDM) has developed as a method for reliable, high-volume data transferin both wire and wireless mediums that can require simpler devicearchitecture than other methods. Wideband digital applications such asdigital television, audio broadcasting, wireless networking, andbroadband internet have become popular applications for OFDMtransmission.

Generally, in OFDM transmission, when the transmission channel is fixedin time and there is no Doppler effect, cross-talk between subchannelscan be eliminated by selecting subcarrier frequencies so thatsubcarriers are orthogonal to each other. Hence, data sent through suchOFDM transmission can be recovered from a received signal by estimatingthe channel in each subcarrier and compensating the signal for theestimated channel without taking into account inter-carrier interference(ICI).

However, in certain cases, such as when the Doppler Effect is present,subcarrier frequencies can become distorted. As a result, thesubcarriers may no longer be orthogonal, resulting in cross-talk betweensubchannels and inter-carrier interference. In these situations, methodsfor data recovery that do not take into account inter-carrierinterference can be insufficient to retrieve the transmitted data. Whatis needed is a system and method that estimates the channel taking intoaccount inter-carrier interference so that channel equalization cancompensate for inter-carrier interference based on the estimatedchannel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of an OFDM signal with scatter pilots,which can be similar to signals in the DVB-T/H and ISDB-T standards.

FIG. 2 illustrates an example of an OFDM receiver with a ChannelEstimation and a Channel Equalization module, in accordance with variousembodiments.

FIG. 3 illustrates an example of a Channel Estimation and a ChannelEqualization module, in accordance with various embodiments.

FIG. 4 illustrates an example of channel estimation in an OFDMtransmission.

FIG. 5 illustrates an example of an OFDM symbol with dedicated datasubcarriers omitted for the sake of illustration.

FIG. 6 illustrates a graphical example of the impulse response of afilter that can be implemented to perform interpolation, in accordancewith various embodiments.

FIG. 7 illustrates an example calculation of a channel in a subcarrierin a shared subchannel to the right of a pilot subcarrier by splineinterpolation that can be implemented in a filter, in accordance withvarious embodiments.

FIG. 8 illustrates an example calculation of a channel in a subcarrierin a shared subchannel between two successive shared subchannels byspline interpolation that can be implemented in a filter, in accordancewith various embodiments.

FIG. 9 illustrates an example calculation of a channel in a subcarrierin a shared subchannel to the left of a pilot subcarrier by splineinterpolation that can be implemented in a filter, in accordance withvarious embodiments.

FIG. 10 illustrates an example calculation of the channel in asubcarrier when a pilot subcarrier is missing, in accordance withvarious embodiments.

FIG. 11 illustrates further examples of channel estimation with missingpilot subcarriers, in accordance with various embodiments.

FIG. 12 illustrates further examples of channel estimation with missingpilot subcarriers, in accordance with various embodiments.

FIG. 13 illustrates further examples of channel estimation with missingpilot subcarriers, in accordance with various embodiments.

FIG. 14 illustrates further examples of channel estimation with missingpilot subcarriers, in accordance with various embodiments.

FIG. 15 illustrates further examples of channel estimation with missingpilot subcarriers, in accordance with various embodiments.

FIG. 16 illustrates further examples of channel estimation with missingpilot subcarriers, in accordance with various embodiments.

FIG. 17 illustrates further examples of channel estimation with missingpilot subcarriers, in accordance with various embodiments.

FIG. 18 illustrates further examples of channel estimation with missingpilot subcarriers, in accordance with various embodiments.

FIG. 19 illustrates further examples of channel estimation with missingpilot subcarriers, in accordance with various embodiments.

FIG. 20A illustrates an example estimation of a channel in a datasubcarrier in a shared subchannel based on an estimate of the channel ina subcarrier in the subchannel of the previous OFDM symbol and anestimate of the channel in the subcarrier in the subchannel of the nextOFDM symbol, in accordance with various embodiments.

FIG. 20B illustrates an example estimation of a channel in a datasubcarrier in a shared subchannel based on an estimate of the channel ina subcarrier in the subchannel of the previous OFDM symbol and anestimate of the channel in the subcarrier in the subchannel of the nextOFDM symbol, when the subcarrier in the subchannel of the next OFDMsymbol is a pilot subcarrier, in accordance with various embodiments.

FIG. 20C illustrates an example estimation of a channel in a datasubcarrier in a shared subchannel based on an estimate of a channel in asubcarrier in the subchannel of the previous OFDM symbol and an estimateof the channel in the subcarrier in the subchannel of the next OFDMsymbol, when the subcarrier in the subchannel of the previous OFDMsymbol is a pilot subcarrier, in accordance with various embodiments.

FIG. 21A illustrates an example of channel estimation in a datasubcarrier in a dedicated data subchannel through the weighted sum ofthe channel in two nearest estimated subcarriers, in accordance withvarious embodiments.

FIG. 21B illustrates an example of channel estimation in a datasubcarrier in a dedicated data subchannel through the weighted sum ofthe channel in two nearest estimated pilot subcarriers, in accordancewith various embodiments.

FIG. 21C illustrates an example of channel estimation in a datasubcarrier in a dedicated data subchannel through the weighted sum ofthe channel in two nearest estimated data subcarriers, in accordancewith various embodiments.

FIG. 21D illustrates an example of channel estimation in a datasubcarrier in a dedicated data subchannel through the weighted sum ofthe channel in a nearest estimated data subcarrier and a nearestestimated pilot subcarrier, in accordance with various embodiments.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the present invention.However, it will be apparent to one skilled in the art that the presentinvention can be practiced without these specific details. In otherinstances, well known circuits, components, algorithms, and processeshave not been shown in detail or have been illustrated in schematic orblock diagram form in order not to obscure the present invention inunnecessary detail. Additionally, for the most part, details concerningcommunication systems, transmitters, receivers, communication devices,computers, and the like have been omitted inasmuch as such details arenot considered necessary to obtain a complete understanding of thepresent invention and are considered to be within the understanding ofpersons of ordinary skill in the relevant art. It is further noted that,where feasible, all functions described herein may be performed ineither hardware, software, firmware, analog components or a combinationthereof, unless indicated otherwise. Certain term are used throughoutthe following description and claims to refer to particular systemcomponents. As one skilled in the art will appreciate, components may bereferred to by different names. This document does not intend todistinguish between components that differ in name, but not function. Inthe following discussion and in the Claims, the terms “including” and“comprising” are used in an open-ended fashion, and thus should beinterpreted to mean “including, but not limited to . . . ”

Embodiments of the present invention are described herein. Those ofordinary skill in the art will realize that the following detaileddescription of the present invention is illustrative only and is notintended to be in any way limiting. Other embodiments of the presentinvention will readily suggest themselves to such skilled persons havingthe benefit of this disclosure. Reference will be made in detail toimplementations of the present invention as illustrated in theaccompanying drawings. The same reference indicators will be usedthroughout the drawings and the following detailed description to referto the same or like parts.

In the interest of clarity, not all of the routine features of theimplementations described herein are shown and described. It will, ofcourse, be appreciated that in the development of any such actualimplementation, numerous implementation-specific decisions must be madein order to achieve the developer's specific goals, such as compliancewith applications and business-related constraints, and that thesespecific goals will vary from one implementation to another and from onedeveloper to another. Moreover, it will be appreciated that such adevelopment effort might be complex and time-consuming, but wouldnevertheless be a routine undertaking of engineering for those ofordinary skill in the art having the benefit of this disclosure.

In various embodiments, systems and methods are described for estimatinga channel in OFDM transmission containing scatter pilot signals.Generally, OFDM signals can be sent over a wire medium, such as cabling,or wireless medium, such as air. When the signal travels through amedium, it may become affected by the medium. The effect can be referredto as the “channel.” By estimating the channel and applying an inversefunction of the channel to a received signal, an original transmittedsignal can be recovered from an affected received signal; in otherwords, the transmitted data can be equalized.

FIG. 1 illustrates an example of an OFDM signal with scatter pilots,which can be similar to signals in the DVB-T/H and ISDB-T standards.Each horizontal row of circles represents an OFDM symbol. Each verticalrow of circles represents a subchannel. Each circle in the figure is asubcarrier, which is a subchannel of an OFDM symbol. The frequency ofthe subchannels can increase from left to right. Subchannels thatcontain only pilot subcarriers 105 are continuous pilot subchannels 101and pilot subcarriers in continuous pilot subchannels 101 are continuouspilot subcarriers 108. Subchannels that contain pilot subcarriers 105and data subcarriers 106 are shared subchannels 103. Data subcarriers inshared subchannels 106 are represented by white circles with a boldoutline in FIG. 1. Scatter pilot subcarriers 104 are pilot subcarriers105 in shared subchannels 103. Dedicated data subchannels 102 containonly data subcarriers 107, represented by white circles with non-boldoutline in the figure.

FIG. 1 illustrates a general example of an OFDM signal. Various types ofOFDM signals in various standards may contain different numbers andconfigurations of pilot subcarriers, data subcarriers, dedicated datasubchannels, shared subchannels, continuous pilot subchannels and othercomponents than illustrated in FIG. 1. As one skilled in the art wouldappreciate, this disclosure is not limited to any particularconfiguration or type of OFDM signal.

In various embodiments, Channel Estimation can be performed to measurethe channel in an OFDM transmission. The channel in an OFDM transmissioncan be estimated by estimating the channel in the subcarriers. Namely,each transmitted subcarrier can be affected by the environment and themedium through which the signal travels when the signal is transmitted.The channel in each subcarrier is effect that such transmission has hadon the data in the subcarrier. In various embodiments, the channels insubcarriers in an OFDM signal can be estimated based on pilotsubcarriers. The channel in pilot subcarriers can be estimated at thereceiver based on a measured subcarrier value and an expected, knownsubcarrier value. The channel in the remaining, non-pilot subcarrierscan be estimated based on the estimated channel in pilot subcarriers.Thus, the channel can be estimated in every subcarrier of every OFDMsymbol. Once the channel is estimated, Channel Equalization can beperformed to compensate the data signals for the channel to recoveroriginal transmitted data.

FIG. 2 illustrates an example of an OFDM receiver with a ChannelEstimation and a Channel Equalization module, in accordance with variousembodiments. A signal can be received through an antenna 201. The signalcan be conveyed to a tuner 202, where the signal can be amplified,filtered, and/or down-converted, for example to a baseband orintermediate frequency (IF). After the tuner 202, the signal can beconveyed to an Analog to Digital Converter (ADC) 203 for analog todigital conversion. The signal can then be conveyed to a filtering andsynchronization module 204 for further filtering and synchronization andthen to a Fast Fourier Transform (FFT) module 300. The FFT module 300can output OFDM symbols. The OFDM symbols can be conveyed to a ChannelEstimation module 301, where the channel can be estimated based on thepilot signals. OFDM symbols, the estimated channel, as well as noisepower data from a Noise Estimator module 303 can be conveyed to theChannel Equalization module 302, where equalized data values can beproduced. The equalized data values can then be conveyed to a DataDemapper 304 and to a Decoder 205 for further processing.

FIG. 3 illustrates an example of a Channel Estimation and a ChannelEqualization module, in accordance with various embodiments. OFDMsymbols can be conveyed from the FFT module 300. Pilot subcarrier valuescan be conveyed to the Channel Estimation module 301 and data subcarriervalues can be conveyed to the Channel Equalization module 302. In theChannel Estimation module 301, the channel in subcarriers can beestimated based on the pilot subcarrier values, as will be described ingreater detail below. In various embodiments, a channel in everysubcarrier, in every OFDM symbol can be estimated. The estimated channelvalues from the channel estimation module 301, data subcarrier valuesfrom the FFT module 300, and noise power data per OFDM symbol from anoise power estimation module 303 can be conveyed to the ChannelEqualization module 302. In the Channel Equalization module 302, datasubcarrier values can be adjusted, or “equalized”, based on theestimated channel in each subcarrier value and the noise power data. Theequalized data values can be conveyed to a Data Demapper 304 for furtherprocessing.

Generally, the channel in a pilot subcarrier can be estimated directlybased on the expected value of the pilot subcarrier and the measuredvalue of the pilot subcarrier. For example, the channel in a pilotsubcarrier can be estimated by dividing the measured value of thesubcarrier by the expected value of the subcarrier. The channel in datasubcarriers in dedicated data subchannels and shared subchannels cannotbe estimated directly, as it can be in pilot subcarriers, because theexpected value in those subcarriers is not known at the receiver.However, in data subcarriers, the channel can be estimated based on theestimated channel in nearby pilot subcarriers.

FIG. 4 illustrates an example of channel estimation in an OFDMtransmission, in accordance with various embodiments. In a transmissionwith a time-invariant channel; that is, where the channel substantiallydoes not change over time, the channel in each subcarrier of asubchannel will be constant. However, the channel may differ acrossdifferent channels. For instance, in the illustrated example, if thechannel in the transmission is time invariant, then the channel in thesubchannel 406 will be constant, in other words, the channel in eachsubcarrier of the subchannel 406 will be substantially the same. Hence,the channel in each of the four illustrated subcarriers of thesubchannel will be approximately the same. In such a scenario, thechannel in the pilot subcarrier 403 can be an accurate estimate of thechannel in the data subcarrier 401. Hence, in a time invariant channel,an estimate of a channel in a pilot subcarrier can be used to estimatethe channel in another data subcarrier in the same subchannel.

However, if the channel in a transmission is time-variant, then thechannel in a subchannel can change from one OFDM symbol to the next. Forexample, this can be the case when the Doppler Effect is present. Insuch cases, the estimated channel in a pilot subcarrier in a subchannel,such as the pilot subcarrier 403 in FIG. 4, may provide a poor estimateof the channel in another data subcarrier in the subchannel, such as thedata subcarrier 401. However, because adjacent parts of each symbol canbe received near in time to each other, time-variant effects can besubstantially consistent throughout a symbol or portions of a symbol. Inthis case, estimated channels in pilot subcarriers in an OFDM symbol canbe used to produce an estimate of the channel in a data subcarrier inthe OFDM symbol. Therefore, when the channel is time-variant, thechannel in a subcarrier can be estimated based on the channel inneighboring subcarriers, such as the channel in neighboring pilotsubcarriers. For example, an estimate of the channel in any of the pilotsubcarriers 402, 404, 405, and 407 in the OFDM symbol in FIG. 4 can beused to produce an estimate of the channel in the data subcarrier 401.Furthermore, in various embodiments, a channel in a subcarrier can beestimated based on both an estimated channel or channels in pilotsubcarriers in the same subchannel as the subcarrier but in differentOFDM symbol(s) and an estimated channel or channels in pilot subcarriersin the same OFDM symbol as the subcarrier.

In various embodiments, the channel in data subcarriers in sharedsubcarriers of an OFDM symbol 106 can be estimated using interpolationmethods based on estimated channels in continuous 108 and scatter pilotsubcarriers 104 in the same OFDM symbol. Linear interpolation can beused to estimate the channel in the data subcarriers in sharedsubchannels 106. In an embodiment, Spline interpolation can be used toestimate the channel in such data subcarriers 106. An interpolationfunction, such as Spline interpolation, can be implemented as a filterto estimate the channel.

FIG. 5 illustrates an example of an OFDM symbol with dedicated datasubcarriers omitted for the sake of illustration. In the illustratedexample, the black circles can represent pilot subcarriers in sharedand/or continuous pilot subchannels 501. The channel in the pilotsubcarriers can be estimated based on expected values, for example bydividing the measure value of the subcarrier by the expected value ofthe subcarrier. In the illustrated example, the white circles canrepresent data subcarriers in shared subchannels 502. The channel in thedata signals 502 can be estimated using methods of interpolation basedon the estimated channels in pilot signals 501.

Interpolation can be implemented through a filter with a desired impulseresponse. In an embodiment, spline interpolation can be implementedthrough a filter. FIG. 6 illustrates a graphical example of the impulseresponse of a filter that can be implemented to perform interpolation.Such a filter can be implemented to estimate the channel in a datasubcarrier 502 of an OFDM symbol such as illustrated in FIG. 5. Thechannel in pilot subcarriers 501 can be estimated based on the expectedand measured values, as described, and the channel can be transferred toan output without change in the filter. Each peak in the graph cancorrespond to a data subcarrier in a shared subchannel 502, such as thesubcarriers illustrated in FIG. 5. The height of each peak, measured onthe Y-axis, can correspond to the weight that can be attributed to acorresponding subcarrier value by the filter, as will be illustratedfurther below.

FIG. 7 illustrates an example calculation of a channel in a subcarrierin a shared subchannel to the right of a pilot subcarrier by splineinterpolation that can be implemented in a filter, in accordance withvarious embodiments. In the OFDM symbol of the illustrated figure, thepure data subcarriers were omitted, as was illustrated in FIG. 5, forthe sake of illustration. Hence, in the illustrated example, the blackcircles can represent pilot subcarriers in shared and/or continuouspilot subchannels 501 and the white circles can represent datasubcarriers in shared subchannels 502. As illustrated in the figure, thechannel in a subcarrier 701 to the right of a pilot subcarrier 703 canbe estimated based on the estimated channel in pilot subcarriers 700,702, 703, 704, 705, and 706 in the OFDM symbol. The channel in the pilotsubcarriers can be estimated based on the expected pilot values and themeasured pilot values. The estimated channel in each pilot subcarriercan be multiplied by a corresponding weight 709, the products can thenbe added in an adder 707, and the sum can be divided by a total weight710. The weights can be applied according to the impulse response asillustrated in FIG. 6. Namely, the pilot subcarrier 703 to the left ofthe subcarrier 701 can be multiplied by the weight corresponding to thepeak 601 to the left of the center peak 600 in the graph of FIG. 6, or905. The pilot subcarrier three carriers to the right 704 of thesubcarrier 701 can be multiplied by the weight corresponding to the peak602 three peaks to the right of the center peak 600 in the graph of FIG.6, or 285. The weights can be similarly assigned to the other pilotsubcarriers 700, 702, 705, and 706.

The spline interpolation can be implemented for other data subcarriersin shared subchannels in an OFDM symbol by implementing the samemethodology as illustrated in FIG. 7.

FIG. 8 illustrates an example calculation of a channel in a subcarrierin a shared subchannel between two successive shared subchannels byspline interpolation that can be implemented in a filter, in accordancewith various embodiments. In the example illustrated in the FIG. 8, themethodology illustrated in FIG. 7 can be implemented to estimate thechannel in a subcarrier 801 between two data subcarriers. The estimatedchannels in pilot subcarriers can be multiplied by corresponding weightsassigned by the spline interpolation method, the products can be added,and the sum can be divided by a total weight as was illustrated in FIG.7. The weights can be chosen according to the distribution illustratedin FIG. 6, according to the same methodology as was described in FIG. 7.

FIG. 9 illustrates an example calculation of a channel in a subcarrierin a shared subchannel to the left of a pilot subcarrier by splineinterpolation that can be implemented in a filter, in accordance withvarious embodiments.

If a pilot subcarrier that is supposed to be used in an estimate ismissing, for example, when a channel in a subcarrier near the edge of anOFDM symbol is estimated, the weight of the missing pilot subcarrier canbe applied to the closest existing pilot subcarrier. FIG. 10 illustratesan example calculation of the channel in a subcarrier when a pilotsubcarrier is missing, in accordance with various embodiments. Asillustrated in the example, because the subcarrier corresponding to the“33 weight” 1001 is missing, the estimated channel in the first pilotsubcarrier 1002 can be used instead of the missing subcarrier. Hence,the estimated channel in the first pilot subcarrier 1002 can be usedtwice in the calculation of the channel in the subcarrier 1003, oncewith the “33 weight” and once with the “−136 weight” 1004.

Similarly, if two or more pilot subcarrier values are missing, then theweight of the missing pilot subcarriers can be applied to the closestexisting pilot subcarrier. FIGS. 11 through 19 illustrate furtherexamples of channel estimation with missing pilot subcarriers, inaccordance with various embodiments.

The above examples illustrate channel estimation implementing splineinterpolation with provided weights. It will be apparent to one ofreasonable skill in the art that channel estimation can be performedusing different methods of interpolation and/or different weights thanthe described examples without departing from the spirit of theinvention. For example, a filter can be implemented to estimate achannel in a subcarrier through linear interpolation, through splineinterpolation with different parameters than described above, throughinterpolation with weights that are selected manually, and through othermethods.

FIG. 20A illustrates an example estimation of a channel in a datasubcarrier in a shared subchannel based on an estimate of the channel ina subcarrier in the subchannel of the previous OFDM symbol and anestimate of the channel in the subcarrier in the subchannel of the nextOFDM symbol, in accordance with various embodiments. As illustrated inthe example of the figure, the channel in a data subcarrier 2000 can beestimated based on the subcarrier 2001 in the subchannel of the previousOFDM symbol and the subcarrier 2002 in the subchannel of the next OFDMsymbol. The channel in the data subcarrier 2000 can be estimated byaveraging the estimated channels in the previous subcarrier 2001 and thenext subcarrier 2002 by adding the estimated channels 2001, 2002 in anadder 2003 and dividing the sum by 2 in a divider 2004. In variousembodiments, different methods can be used to estimate the subcarrier2000 based on the estimated channels in the previous subcarrier 2001 andthe next subcarrier 2002. For example a linear combination of theestimated channels 2001, 2002 can be used to estimate the channel 2000with more weight being placed on one estimated channel than the other,for example a weight of 0.4 can be placed on the channel estimate in theprevious subcarrier 2001 and a weight of 0.6 can be placed on thechannel estimate in the next subcarrier 2002. The estimate of thechannel in subcarriers 2001 and 2002 can be performed based on pilotsubcarriers. In various embodiments, the channel in the subcarriers 2001and 2002 can be estimated according to methods described above, forexample, through the spline interpolation method described above.

FIG. 20B illustrates an example estimation of a channel in a datasubcarrier in a shared subchannel based on an estimate of the channel ina subcarrier in the subchannel of the previous OFDM symbol and anestimate of channel in the subcarrier in the subchannel of the next OFDMsymbol, when the subcarrier in the subchannel of the next OFDM symbol isa pilot subcarrier, in accordance with various embodiments. In such acase, an estimate of the channel 2005 can be performed as describedabove in FIG. 20A. The channel in the pilot subcarrier 2006 can beestimated based on the expected value of the subcarrier 2006 and themeasured value of the subcarrier 2006, as described above.

FIG. 20C illustrates an example estimation of a channel in a datasubcarrier in a shared subchannel based on an estimate of a channel in asubcarrier in the subchannel of the previous OFDM symbol and an estimateof the channel in the subcarrier in the subchannel of the next OFDMsymbol, when the subcarrier in the subchannel of the previous OFDMsymbol is a pilot subcarrier, in accordance with various embodiments. Insuch a case, an estimate of the channel 2007 can be performed asdescribed above in FIG. 20A. The channel in the pilot subcarrier 2008can be estimated based on the expected value in the subcarrier 2008 andthe measured value in the subcarrier 2008, as described above.

FIGS. 20A through 20B illustrate using subcarriers in the samesubchannel of the immediately previous OFDM symbol and the immediatelysucceeding OFDM symbol to estimate the channel in a subcarrier. Invarious other embodiments, subcarriers in the same subchannel of otherpreceding and/or succeeding OFDM symbols can be used instead of or inaddition to the immediately previous and immediately next OFDM symbolsto estimate the channel in a subcarrier.

In various embodiments, a channel in a data subcarrier can be estimatedby combining two different estimates of the channel in the datasubcarrier. For example, an estimate of the channel in the subcarrierobtained based on pilot signals in the OFDM symbol, for example asillustrated above in FIGS. 7 through 19, can be combined with anestimate of the channel in the same subcarrier obtained based on anestimate of a subcarrier channel in the subchannel of the previous OFDMsymbol and an estimate of the subcarrier channel in the subchannel ofthe next OFDM symbol, as illustrated in FIGS. 20A through 20C. In anembodiment, the two estimates can be combined in a linear combinationwith a coefficient of ½. For example, the first estimate can bemultiplied by ½ and the second estimate can be multiplied by ½ and thetwo products can be added. In other embodiments, the multiples can bedifferent to give more weight to one estimate that the other.

Thus, in an embodiment, a channel in a subcarrier can be first estimatedbased on pilot subcarriers in the OFDM symbol using splineinterpolation, for example as illustrated above in FIGS. 7 through 19.The channel in the same subcarrier can be estimated a second time bycombining an estimate of the subcarrier in the same subchannel of theprevious OFDM symbol and an estimate of the subcarrier in the samesubchannel of the next OFDM symbol, for example as illustrated in FIGS.20A through 20C. The estimate of the subcarrier in the subchannel of theprevious OFDM symbol and the estimate of the subcarrier in thesubchannel of the next OFDM symbol can be obtained by splineinterpolation based on pilot subcarriers in the OFDM symbol or, if thesubcarrier is a pilot subcarrier, based on the expected pilot value. Athird estimate of the subchannel can then be produced by combining thefirst and the second estimates of the channel in the subcarrier, forexample in a linear combination with a coefficient of ½.

In various embodiments, the channel in each of the data subcarriers 107in dedicated data subchannels 102 as illustrate in FIG. 1, which wereomitted in the illustration of FIG. 5, can be estimated based on theestimated channel in the two nearest subcarriers. FIG. 21A illustratesan example of channel estimation in a data subcarrier in a dedicateddata subchannel through the weighted sum of the channel in two nearestestimated subcarriers. In the example illustrated, the gray circle is adata subcarrier in a shared subchannel 2101; the black circle is a pilotsubcarrier in a shared or a continuous subchannel 2104; the whitecircles are data subcarriers in dedicated data subchannels 2102, 2103.To estimate the channel in the data subcarrier 2103, the estimatedchannel in the data subcarrier 2101 can be multiplied by 5 in amultiplier 2105 and the estimated channel in the pilot subcarrier 2104can be multiplied by 11 in a multiplier 2106. The products can be addedin an adder 2107 and divided by 16 in a divider 2108 to estimate thechannel in the data subcarrier 2103. The channel in the subcarrier 2102can be similarly estimated by producing a weighted sum with a weight of11 being applied to the data subcarrier 2101 and a weight of 5 beingapplied to the pilot subcarrier 2104. The channel in the data subcarrier2101 and the pilot subcarrier 2104 can be estimated based on the methodsdescribed above in the specification.

FIG. 21B illustrates an example of channel estimation in a datasubcarrier in a dedicated data subchannel through the weighted sum ofthe channel in two nearest estimated pilot subcarriers, in accordancewith various embodiments.

FIG. 21C illustrates an example of channel estimation in a datasubcarrier in a dedicated data subchannel through the weighted sum ofthe channel in two nearest estimated data subcarriers, in accordancewith various embodiments.

FIG. 21D illustrates an example of channel estimation in a datasubcarrier in a dedicated data subchannel through the weighted sum ofthe channel in a nearest estimated data subcarrier and a nearestestimated pilot subcarrier, in accordance with various embodiments.

In various embodiments, the channel in each of the data subcarriers 107in dedicated data subchannels 102, as illustrated in FIG. 1, can beestimated in a linear combination of the estimated channels in the twonearest subcarriers as illustrated in the examples in FIGS. 21B through21C. In various embodiments, the linear combination, as illustrated inthe examples in FIGS. 21B through 21C, can be performed using differentweights in the linear combination than illustrated. In variousembodiments, different methods, such as non-linear combination, can beused to estimate the channel in each of the data subcarriers 107 indedicated data subchannels 102 based on the estimated channels in thetwo nearest subcarriers.

Embodiments of the system and method described herein facilitateestimating the channel in OFDM transmission. Although the components andmodules illustrated herein are shown and described in a particulararrangement, the arrangement of components and modules may be altered toperform the described functions and processes in a different manner. Inother embodiments, one or more additional components or modules may beadded to the described systems, and one or more components or modulesmay be removed from the described systems. Alternate embodiments maycombine two or more of the described components or modules into a singlecomponent or module.

While certain exemplary embodiments have been described and shown in theaccompanying drawings, it is to be understood that such embodiments aremerely illustrative of and not restrictive on the broad invention, andthat this invention is not limited to the specific constructions andarrangements shown and described, since various other modifications mayoccur to those ordinarily skilled in the art. Accordingly, thespecification and drawings are to be regarded in an illustrative ratherthan a restrictive sense.

Reference in the specification to “an embodiment,” “one embodiment,”“some embodiments,” “various embodiments” or “other embodiments” meansthat a particular feature, structure, or characteristic described inconnection with the embodiments is included in at least someembodiments, but not necessarily all embodiments. References to “anembodiment,” “one embodiment,” or “some embodiments” are not necessarilyall referring to the same embodiments. If the specification states acomponent, feature, structure, or characteristic “may,” “can,” “might,”or “could” be included, that particular component, feature, structure,or characteristic is not required to be included. If the specificationor Claims refer to “a” or “an” element, that does not mean there is onlyone of the element. If the specification or Claims refer to an“additional” element, that does not preclude there being more than oneof the additional element.

The invention claimed is:
 1. A method comprising: receiving anOrthogonal Frequency-Division Multiplexing (OFDM) signal with scatterpilots; and producing a first estimate of a channel in a first estimateddata subcarrier in a subchannel in an OFDM symbol of the OFDM signalthrough interpolation, the interpolation being based on estimatedchannels in both of: (a) at least one pilot subcarrier that is in thesame subchannel as the data subcarrier but in a different OFDM symbolthan the data subcarrier; and (b) at least two pilot subcarriers thatare in the same OFDM symbol as the data subcarrier but a differentsubchannel than the data subcarrier, wherein the estimated channels inthe at least two pilot subcarriers in the same OFDM symbol aredetermined by: assigning a corresponding weight to each of the at leasttwo pilot subcarriers that are in the same OFDM symbol as the datasubcarrier but the different subchannel than the data subcarrier basedon impulse response of a filter; in response to detecting that one ofthe at least two pilot subcarriers is missing, assigning thecorresponding weight of the missing pilot subcarrier to a closestexisting pilot subcarrier of the at least two pilot subcarriers, suchthat the closest existing pilot subcarrier is applied at least twicewith two different weights to determine the estimated channel;multiplying each of the at least two pilot subcarriers by thecorresponding weight to compute two or more products; adding the two ormore products in an adder to compute a sum; and dividing the sum by atotal weight assigned to all of the at least two pilot subcarriers. 2.The method of claim 1, wherein the interpolation is based on estimatedchannels in at least three pilot subcarriers in the OFDM symbol.
 3. Themethod of claim 1, wherein the first estimate of the channel in thefirst estimated data subcarrier is produced through Splineinterpolation.
 4. The method of claim 1, further comprising: producing asecond estimate of the channel in the first estimated data subcarrierbased on an estimate of the channel in a subcarrier in the samesubchannel in a previous OFDM symbol and an estimate of the channel in asubcarrier in the same subchannel in a succeeding OFDM symbol.
 5. Themethod of claim 4, wherein the second estimate of the channel in thefirst estimated data subcarrier is produced by taking an average of anestimated channel in a subcarrier in the same subchannel in a previousOFDM symbol and an estimate of a channel in a subcarrier in the samesubchannel in a succeeding OFDM symbol.
 6. The method of claim 4,further comprising: producing a third estimate of the channel in thefirst estimated data subcarrier based on the first estimate of thechannel in the first estimated data subcarrier and the second estimateof the channel in the first estimated data subcarrier.
 7. The method ofclaim 6, wherein the third estimate of the channel in the firstestimated data subcarrier is produced through a linear combination ofthe first estimate of the channel in the first estimated data subcarrierand the second estimate of the channel in the first estimated datasubcarrier.
 8. The method of claim 6, further comprising estimating achannel in a data subcarrier in a designated data subchannel based onthe estimated channels in two nearest subcarriers in shared subchannels,wherein the two nearest subcarricrs in shared subchannels are in thesame OFDM symbol as the data subcarrier in the designated datasubchannel.
 9. The method of claim 6, further comprising estimating achannel in a data subcarrier in a designated data subchannel through aweighted sum of the estimated channels in two nearest subcarriers inshared subchannels, which two nearest subcarriers in shared subchannelsare in the same OFDM symbol as the data subcarrier in the designateddata subchannel.
 10. The method of claim 1, wherein the correspondingweight is based on a peak in the impulse response of the filtercorresponding to the pilot signal received by the filter.
 11. Anapparatus comprising: an antenna configured to receive an OrthogonalFrequency-Division Multiplexing (OFDM) signal with scattered pilots; anda Channel Estimator configured to produce a first estimate of a channelin a first estimated data subcarrier in a shared subchannel in an OFDMsymbol of the received OFDM signal through interpolation. theinterpolation being based at least in part on estimated channels in bothof: (a) at least one pilot subcarrier in the same subchannel as the datasubcarrier but a different OFDM symbol than the data subcarrier; and (b)at least two pilot subcarriers in the same OFDM symbol as the datasubcarrier but a different subchannel than the data subcarrier, whereinthe estimated channels in the at least two pilot subcarriers in the sameOFDM symbol are determined by: assigning a corresponding weight to eachof the at least two pilot subcarriers that are in the same OFDM symbolas the data subcarrier but the different subchannel than the datasubcarrier, wherein the corresponding weight is based at least in parton an impulse response of a filter; in response to detecting that one ofthe at least two pilot subcarriers is missing, assigning thecorresponding weight of the missing pilot subcarrier to a closestexisting pilot subcarrier of the at least two pilot subcarriers, suchthat the closest existing pilot subcarrier is applied at least twicewith two different weights to determine the estimated channel;multiplying each of the at least two pilot subcarriers by thecorresponding weight to compute two or more products; adding the two ormore products in an adder to compute a sum; and dividing the sum by atotal weight assigned to all of the at least two pilot subcarriers. 12.The apparatus of claim 11, wherein the Channel Estimator is furtherconfigured to produce the first estimate of the channel in the firstestimated data subcarrier in the shared subchannel through Splineinterpolation.
 13. The apparatus of claim 11, wherein the ChannelEstimator is further configured to: produce a second estimate of thechannel in the first estimated data subcarrier in the shared subchannelbased on an estimate of a channel in a subcarrier in a previous OFDMsymbol in the same subchannel and an estimate of a channel in asubcarrier in a succeeding OFDM symbol in the same subchannel.
 14. Theapparatus of claim 13, wherein the Channel Estimator is furtherconfigured such that the second estimate of the channel in the firstestimated data subcarrier in the shared subchannel is produced by takingan average of the estimate of the channel in the subcarrier in theprevious OFDM symbol in the same subchannel and the estimate of thechannel in the subcarrier in the succeeding OFDM symbol in the samesubchannel.
 15. The apparatus of claim 13, wherein the Channel Estimatoris further configured to produce a third estimate of the channel in thefirst estimated data subcarrier in the shared subchannel based on thefirst estimate of the first estimated data subcarrier in the sharedsubchannel and the second estimate of the first estimated datasubcarrier in the shared subchannel.
 16. The apparatus of claim 15wherein the Channel Estimator is further configured to estimate achannel in a data subcarrier in a designated data subchannel based onthe estimated channel of two nearest subcarriers in shared subchannelsin the same OFDM symbol.
 17. The apparatus of claim 15 wherein theChannel Estimator is further configured to estimate a channel in a datasubcarrier in a designated data subchannel through a weighted sum of thechannels in two nearest subcarriers in shared subchannels in the sameOFDM symbol.
 18. The apparatus of claim 13 wherein the Channel Estimatoris further configured such that the third estimate of the channel in thefirst estimated data subcarrier in the shared subchannel is producedthrough a linear combination of the first estimate of the channel in thefirst estimated data subcarrier in the shared subchannel and the secondestimate of the channel in the first estimated data subcarrier in theshared subchannel.
 19. An Orthogonal Frequency-Division Multiplexing(OFDM) receiver, comprising: an antenna configured to receive an OFDMsignal with scattered pilots; a tuner configured to perform at least oneof: amplify, filter or down-convert the OFDM signal; ananalog-to-digital converter (ADC) configured to convert the OFDM signalto digital form; a filtering and synchronization module configured tofilter and synchronize the OFDM signal; a Fast Fourier Transform (FFT)module configured to output OFDM symbols; a Channel Estimator configuredto produce a first estimate of a channel in a first estimated datasubcarrier in a shared subchannel in an OFDM symbol of the received OFDMsignal through interpolation, the interpolation being based at least inpart on estimated channels in both of: (a) at least one pilot subcarrierin the same subchannel as the data subcarrier but a different OFDMsymbol than the data subcarrier; and (b) at least two pilot subcarriersin the same OFDM symbol as the data subcarrier but a differentsubchannel than the data subcarrier, wherein the estimated channels inthe at least two pilot subcarriers in the same OFDM symbol aredetermined by: assigning a corresponding weight to each of the at leasttwo pilot subcarriers that are in the same OFDM symbol as the datasubcarrier but the different subchannel than the data subcarrier,wherein the corresponding weight is based at least in part on impulseresponse of a filter; in response to detecting that one of the at leasttwo pilot subcarriers is missing, assigning the corresponding weight ofthe missing pilot subcarrier to a closest existing pilot subcarrier ofthe at least two pilot subcarriers, such that the closest existing pilotsubcarrier is applied at least twice with two different weights todetermine the estimated channel; multiplying each of the at least twopilot subcarriers by the corresponding weight to compute two or moreproducts; adding the two or more products in an adder to compute a sum;and dividing the sum by a total weight assigned to all of the at leasttwo pilot subcarriers.